It lets you track message at certain points in their life cycle by invoking custom code from user exits while the message is passing through the message flow.
它允许您在消息通过消息流传递时调用用户出口中的自定义代码，从而在消息生命周期中的某个点跟踪消息。
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First, it would complicate your code because you would have to manage a pool and track the life cycle of the XHR instances.